1. Field of the Invention
The present invention relates to an impedance matched electrical connector comprising a plug connector member and a jack connector member matable with the plug connector member for providing an electric connection between impedance matched printed wiring boards.
2. Description of the Related Art
With the development of high speed signal processing in computers, it has been required that multielectrode connectors for computers should have a characteristic impedance matched with a characteristic impedance of printed wiring boards and cables to avoid a reflection of signals in the connectors, and that the connectors should provide for a low level of crosstalk between the signals. To meet such requirements for high speed signal processing, it is necessary that the printed wiring boards, wirings of the cables, and terminals in the electrical connectors are not mere electrical connector means, but are also transmission lines. Namely, it is desirable to arrange the signal lines relative to the ground conductor in a constant disposition (or a cross-sectional shape) in order not to disturb electromagnetic waves around the signal lines.
There are mainly two kinds of signal transmissions: (a) an unbalanced transmission that uses a signal conductor and a ground conductor, and (b) a balanced transmission that uses a positive signal conductor and a negative signal conductor. Unbalanced transmission is frequently used in printed circuit boards and electrical casings for digital signal transmissions. The unbalanced transmission is classified into the following three types depending on the disposition of the signal conductor and the ground conductor, i.e., a coaxial structure in which a core signal conductor is surrounded by a cylindrical ground conductor with a dielectric material (insulating material) filled therebetween; a strip line structure in which a signal conductor is sandwiched between a pair of ground planes with a dielectric material filled therebetween, and a microstrip line structure in which a signal conductor is arranged parallel to only one (i.e., a single) ground plane with a dielectric material filled therebetween.
Transmission characteristics vary between the strip line structure and the microstrip line structure. In the strip line structure, the occurrence of noise and crosstalk are relatively low. Signals are less affected by external electromagnetic fields, and the impedance characteristic is stable. In the microstrip line structure, signals are relatively affected by external electromagnetic fields (i.e., the shielding effect is small). The occurrence of noise is high, and the impedance characteristics are unstable. Also, the signals are affected by the electromagnetic fields emitted from the signal conductors themselves, depending on the disposition of the dielectric material and the conductors; further, the occurrence of crosstalk is relatively high.
The respective impedance characteristics, as between the strip line structure and the microstrip line structure, are different, even when the dimensional conditions of the corresponding elements are identical. It is well known that the impedance Z.sub.0 can be calculated by the following relationships, in which e is a dielectric constant of dielectric material; d is a diameter of a signal conductor when it has a circular cross section; and h is a distance between the center of the signal conductor and the ground plane.
The impedance Z.sub.0 of the strip line structure ##EQU1##
The impedance Z.sub.0 of the microstrip line structure is: ##EQU2##
The printed wiring boards are formed in a multi-layered structure having outer conductor layers and an innermost conductor layer with dielectric layers therebetween. The ground conductor and the power supply conductor are usually located on the innermost conductor layer and thus together constitute a microstrip line structure, in which the ground plane is arranged only on one side of the signal conductors.
There are proposals for electrical connectors having an impedance characteristic matching the impedance of the printed wiring boards to which the electrical connector is attached. For example, Japanese Examined Utility Model (Kokoku) 49-6543 discloses an electrical connector for printed wiring boards having a microstrip structure. This electrical connector is shown in FIGS. 8A to 8C of the attached drawings, and comprises a plug connector member 50 and a jack connector member 52 which is matable with the plug connector member 50. FIG. 8B is a perspective, cross-sectional view through the jack connector member 52 and FIG. 8C is a perspective cross-sectional view through the plug connector member 50. The plug connector member 50 comprises a plug housing 54 of insulating material with three rows of holes, in which a first row of male signal terminals 56, a second row of male signal terminals 58 and a generally planar and elongated male conductor 60 are fitted, respectively The elongated male conductor 60 is arranged between the male signal terminals 56 and 58 so that a microstrip line structure is formed. The jack connector member 52 comprises a jack housing 62 of insulating material with three rows of apertures 64, 66 and 68, 64 and 66 comprising plural holes and 69 comprising at least one elongated slot. A generally planar and elongated female conductor 70 is inserted in the central slot 68 to receive the elongated male conductor 60 for mating engagement therewith. Upper and lower female terminals 72 and 74 are inserted in the upper and lower holes 64 and 66, respectively, for mating engagement with the male signal terminals 56 and 58. The elongated female conductor 70 also forms a microstrip line structure with the female signal terminals 72 and 74. A shield case 76 is attached over the jack housing 62. This shield case 76 shields external electromagnetic waves and is not designed to control the impedance, i.e., as a strip line structure.
In this connector, the male conductor 60 has a longer length than that of the male signal terminals 56 and 58 so that the male conductor 60 first engages the central slot 68 of the jack housing 62 to function as a guide when the plug connector member 50 is mated with the jack connector member 52. Also, the male conductor 60 has a central slit 78 that cooperates with a guide wall 80 of the jack housing 62, at the discontinuity of the central slot 68, to function as a guide when the plug connector member 50 is mated with the jack connector member 52.
U.S. Pat. No. 4,762,500 also discloses an impedance matched electrical connector with a microstrip structure. This connector also includes a planar and elongated ground conductor and two rows of signal terminals on either side of the ground conductor to form a microstrip structure.
Recently, multi-layered printed wiring boards have been developed and the number of conductive layers of such printed wiring boards is increasing. Thus the ground conductor and the power supply conductor are not necessarily located on the innermost conductor layer and instead are distributed on several layers so that the signal conductors are arranged between the ground conductor and the power supply conductor or between the ground conductors. In this case, a strip line structure is constituted in which the ground planes are arranged on either side of the signal conductors, since the constant-voltage power supply is deemed to be an equivalent of the ground with regard to high frequency signal processing.
It is possible in principle to design an electrical connector having a microstrip line structure of a particular impedance matched to an impedance of a desired multi-layered printed wiring board. To this end, it is necessary to select the distance h between the center of the signal conductor and the ground plane in relation to the diameter of the signal conductor. In fact, however, when the density of the signal conductors becomes large, the diameter of the signal conductor becomes small and the pitch between the adjacent signal conductors becomes small. There is a problem in the electrical connector having a microstrip line structure of a particular impedance selected under these conditions, in that that the impedance is relatively unstable and the occurrence of crosstalk is high.